Januaey 26, 1900.] 



SCIENCE. 



155 



by the headward growth of the Coosa. Ridges 

 of hard strata, produced by the dissection of an 

 uplifted peneplain of monoclinal structure, are 

 believed to retain their even crests while re- 

 duced toward the new baselevel, and hence 

 should not alone be taken as indicating a pene- 

 plain. Differences of structure are recognized 

 as controlling many local drainage adjustments, 

 but broader modifications of drainage are be- 

 lieved to result from "the slight warping of 

 the land service which appears to accompany 

 all uplifts." 



RIVEB SPACING AND REGIONAL BEVELING. 



A WELCOME continuation of the discussion on 

 peneplanation begun by Tarr {Amer. 6eol., 

 June, 1898) is found in articles by Shaler 

 (Spacing of rivers with reference to hypothesis 

 of baseleveling. Bull. Geol. Soc. Amer., X., 

 1899, 263-276) and Tangier Smith (Some as- 

 pects of erosion in relation to the theory of the 

 peneplain, Univ. Cal.; Bull. Dept. Geol., II., 

 1899 155-178). The first article suggests that 

 the often observed approach to uniformity of 

 interval between adjacent valleys, and to uni- 

 formity in the slope of valley sides ' ' tends to 

 bring about a likeness in the height of the di- 

 vides even where the original surface was of 

 varied elevation" ; and the so-called peneplains 

 of the Appalachian region (such as those of the 

 Chattanooga district, referred to above) are 

 thought to be better accounted for in this way 

 than by baseleveling ; but the best preserved 

 peneplain of this region, that of the Piedmont 

 belt, is regarded as beyond explanation by 

 river and atmospheric action, and is therefore 

 by implication referred to a marine origin. 



The second article deals more elaborately 

 with the development of graded valley sides 

 and with the correlations of summit height, 

 side slope, and stream action in regions of ma- 

 ture dissection. The roughly equal spacing of 

 the principal rivei-s of a topographic unit is said 

 to ' follow as a necessary corollary ' from the 

 general principles thus deduced. Regions ex- 

 hibiting a general uniformity of summit height, 

 but so maturely dissected as no longer to pre- 

 serve remnants of their initial uplands, are re- 

 garded as more probably explained by stream 

 spacing and hill grading in the present cycle, 



than by peneplanation in a former cycle. The 

 beveling of a region by the more rapid degra- 

 dation of the hills near the coast than in the 

 interior, as previously suggested by Tarr and 

 here more fully stated, is held to give suflicient 

 explanation of facts that have been referred by 

 others to the unequal uplift or tilting of a pene- 

 plain. On the other hand, uplands that con- 

 sist of truncated hills of accordant height, 

 capped with residual soils and bearing old river 

 gravels, are regarded as true uplifted pene- 

 plains. 



As to beveling versus tilting, truly the degra- 

 dation of hills must be a little faster near the 

 coast than in the interior, but the excess does 

 not account for the slanting descent of the New 

 England upland southward to the shore of 

 Long Island sound, or for the gradual decrease 

 in height of the Cumberland penelain from Ten- 

 nessee into Alabama. 



AN ANCIENT PLAIN IN COLORADO. 



W. O. Crosby gives a detailed account of the 

 remarkably smooth floor of crystalline rocks on 

 which the Cambrian sandstones rest in the 

 Rocky Mountain front range in Colorado. 

 Although now tilted and more or less deformed, 

 the floor is described as originally of very faint 

 relief, with residual eminences only three or 

 four feet high over areas of many square miles ; 

 but it may be noted that broad swells and 

 troughs are not excluded by any direct evi- 

 dence. Comparing this with other smooth sub- 

 Cambrian floors in the United States, Crosby 

 concludes that they are all parts of an exten- 

 sive surface of planation (abrasion), produced 

 during a period of slow subsidence, by marine 

 attack on a region that ' ' may very well have 

 been reduced to a peneplain by prior subaerial 

 erosion" {Bull. Geol. Soc. Amer. X., 1899, 141- 

 164). 



The occurrence of marine strata on a floor of 

 firm, uuweathered rock certainly points to 

 marine abrasion before deposition began, but 

 it may be urged that the prevailing absence of 

 valleys in the even sub-Cambrian floor suggests 

 the change of ' may very well have been ' to 

 ' must have been ' in the preceding quotation. 

 The broad floor was in any case the result of 

 the destruction of an extensive pre- Cambrian 



